U.S. patent number 3,965,447 [Application Number 05/538,985] was granted by the patent office on 1976-06-22 for synthetic reactor circuit.
This patent grant is currently assigned to GTE Automatic Electric Laboratories Incorporated. Invention is credited to Robert M. Thomas.
United States Patent |
3,965,447 |
Thomas |
June 22, 1976 |
Synthetic reactor circuit
Abstract
Communication system interface circuits for bi-directional
transmission of signals between terminal sets, each interface
circuit comprising a transformer including first winding means
coupled through rectifier means and a low-pass filter to a first
terminal set and including second winding means having end
terminals coupled through diodes to the output of a high frequency
inverter and having a center tap coupled through an inductive
impedance to a neutral circuit point, a second terminal set being
coupled across the inductive impedance. The first terminal set may
be connected to a telephone subset with ringing and bias signals
being applied in circuit with the inductive impedance. The
inductive impedance includes transistor means controlled by an
operational amplifier which is controlled from a
resistance-capacitance phase-shift means, the impedance being
equivalent to that of an inductance and a series resistance.
Inventors: |
Thomas; Robert M. (Brockville,
CA) |
Assignee: |
GTE Automatic Electric Laboratories
Incorporated (Northlake, IL)
|
Family
ID: |
27022988 |
Appl.
No.: |
05/538,985 |
Filed: |
January 6, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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415481 |
Nov 13, 1973 |
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Current U.S.
Class: |
333/214; 330/107;
379/405 |
Current CPC
Class: |
H04B
1/52 (20130101); H04M 19/008 (20130101); H03H
11/485 (20130101) |
Current International
Class: |
H03H
11/48 (20060101); H04B 1/52 (20060101); H03H
11/00 (20060101); H04B 1/50 (20060101); H04M
19/00 (20060101); H03H 011/00 () |
Field of
Search: |
;333/8R,8T
;330/9,107,109 ;307/295,230 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This is a division of application Ser. No. 415,481, filed Nov. 13,
1973.
Claims
I claim as my invention:
1. A synthetic reactor circuit comprising transistor means, an
operational amplifier having an output coupled to said transistor
means for controlling conduction therethrough, and
resistance-capacitance means responding to the voltage across said
transistor means and applying a signal to the non-inverting input
of said operational amplifier in phase-displaced relation from said
voltage such that the effective impedance presented by said
transistor means is a reactive impedance.
2. In a circuit as defined in claim 1, resistance means in series
with said transistor means, said operational amplifier having a
second input responsive to the voltage across said series
resistance means.
Description
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to communications system interface circuits
and more particularly to interface circuits for efficient
transmission of signals such as in an electronic terminating
junctor and has a line interface device between telephone lines and
electronic networks. The circuit of this invention uses inexpensive
and compact parts and is readily and economically
manufacturable.
II. Background of the Prior Art
In communications systems such as telephone systems, it is common
practice to establish a direct metallic path between a pair of
terminal sets for bi-directional transmission of signals
therebetween. With a direct metallic path, undesired signals such
as transient voltage surges are transmitted along with the desired
signals and also the impedances at the interconnected terminal sets
must be equal to obtain an impedance match. Impedance matching is
oftentimes important, especially when establishing interfaces such
as between telephone lines and electronic networks. Conventional
transformers may be used for isolation and also to obtain an
impedance match, but such are bulky and expensive when low
frequency signals such as audio signals are to be transmitted.
SUMMARY OF THE INVENTION
This invention was evolved with the general object of overcoming
the disadvantages of prior art arrangements. A more specific object
of the invention is to provide a synthetic inductor circuit which
simulates the operation of a conventional inductor without
requiring coils wound on a core of material.
In accordance with this invention, a pair of coupling means are
provided for coupling a pair of terminal sets to a pair of
windingmeans of a transformer means to which high frequency pulses
are applied, each coupling means being arranged to develop a signal
at the corresponding terminal set corresponding to the magnitude of
pulses in the corresponding winding means and each coupling means
being also responsive to a signal applied to the corresponding
terminal set to control the magnitude of the pulses in the
corresponding winding means and thereby the magnitude of pulses in
the other of the winding means. Thus signals can be transmitted in
either direction, from either of the terminal sets to the other. At
the same time, isolation is provided, there being no direct
metallic path between the terminal sets. Also, any desired
impedance match may be obtained. A very important advantage is that
since the transformer means is operative in conjunction with high
frequency pulses, the size thereof can be quite small and also the
transformer means can be quite inexpensive.
The high frequency pulses are applied to one of the winding means
which preferably has a center tap coupled to a neutral circuit
point, with current pulses being alternately conducted between the
neutral circuit point and end terminals of the center-tapped
winding means. An inverter-type circuit may be employed, with a
voltage source between the center tap and the neutral circuit point
and with alternately conductive transistors between the neutral
circuit point and the end terminals. Alternatively, a separate
inverter-type circuit may be used, coupled to the neutral circuit
point and through diodes to the end terminals of the center-tapped
winding means.
One of the coupling means includes impedance means coupled between
the center tap of the center-tapped winding and the neutral circuit
point to develop a signal at the correspoinding terminal set
corresponding to the magnitude of pulses in the center-tapped
winding, while being responsive to a signal applied to the
corresponding terminal set to control the magnitude of the pulses
in the center-tapped winding means and thereby the magnitude of the
pulses in the other winding means.
The other coupling means includes rectifier or detector means,
preferably having a full-wave bridge configuration, with lowpass
filter means coupling the output of the rectifier means to the
corresponding terminal set.
An important feature relates to a circuit referred to herein as a
synthetic inductor circuit which uses an operational amplifier,
transistor means and resistance and capacitance components to
provide an impedance equivalent to that of an inductor with a
series resistance.
This invention contemplates other objects, features and advantages
which will become more fully apparent from the following detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a basic inverter circuit for explanation of the theory
of operation of the interface circuits of the invention;
FIG. 2 is a graph showing the relationship between powers in
different portions of the circuit;
FIG. 3 shows a circuit like that of FIG. 1, but with an additional
voltage applied for further explanation of the operation;
FIG. 4 shows an alternative version of the circuit of FIG. 3, with
a common energizing source for a group of circuits;
FIG. 5 is a block diagram illustrating a portion of a telephone
system incorporating interface circuits and other features of the
invention;
FIG. 6 is a schematic diagram of an interface circuit of the system
of FIG. 5;
FIG. 7 is a schematic diagram of an inverter circuit of the system
of FIG. 5;
FIG. 8 is a schematic diagram of a modified circuit arrangement;
and
FIG. 9 is a circuit diagram of a synthetic capacitor used in the
modified arrangement of FIG. 8.
BASIC THEORY
Referring to FIG. 1, reference numeral 10 generally designates a
circuit for explanation of the basic theory of the invention.
Circuit 10 comprises an inverter circuit of the type commonly used
for power conversion and includes a transformer 11 having a primary
winding 12 which has a center tap connected to the positive
terminal of a source 13 of a voltage V.sub.1, the negative terminal
of which is connected to a neutral circuit point 14 and to the
emitters of a pair of transistors 15 and 16 the collectors of which
are connected to end terminals of the primary winding 12. The base
electrodes of transistors 15 and 16 are connected to a base drive
circuit 18 operative to render the transistors 15 and 16
alternately conductive, a voltage 2V.sub.1 being thereby developed
across the primary winding 12.
The transformer 11 has a secondary winding having a center tap
connected to a circuit point 20 and having end terminals connected
through rectifier diodes 21 and 22 to the input of a low pass
filter 23, a load resistor 24 being connected between the circuit
point 20 and the output of the filter 23.
The voltage developed across the secondary winding 19 is 2V.sub.1
N.sub.2 /N.sub.1 and the recrifier output is thereby V.sub.1
N.sub.2 /N.sub.1, N.sub.1 being the number of turns of the primary
winding 12 and N.sub.2 being the number of turns of the secondary
winding 19. The output current is given by the equation: ##EQU1##
R.sub.1 being the resistance of the load resistor 24. Thus the
circuit operates as a transforer with response down to D.C.
Small variations in V.sub.1 (i.e. an audio signal superimposed on
V.sub.1) are also transferred to the secondary side. The
attenuation of audio signals is governed by the incremental power
converstion efficiency of the circuit. With reference to FIG. 2,
line 26 shows the relationship of power P.sub.2 in the secondary
side to power P.sub.1 in the primary side, the overall efficiency
being indicated by dotted line 27. A small change .DELTA. P.sub.1
in power input produces a small change .DELTA. P.sub.2 in output,
the slope of a line 28 being the incremental efficiency
.DELTA.P.sub.2 /.DELTA.P.sub.1.
FIG. 3 shows the same circuit as FIG. 1, with a resistor 29 in
series with the voltage source 13 and with a voltage source 30 in
series with the load resistor 24, producing a small voltage V.sub.2
in the secondary side. The output current is expressed by the
Equation: ##EQU2##
Since I.sub.1 = I.sub.2 N.sub.2 /N.sub.1 , it follows that:
##EQU3##
It can be seen that the circuit operates as a transformer in the
reverse direction provided that the voltages are such that I.sub.2
is greater than zero at all times. Small DC or audio signals can
therefore be passed through the circuit in either direction. It may
also be noted that an alternate proof is possible using the
Reciprocity theorem.
FIG. 4 shows an alternative circuit arrangement 32 wherein one
inverter is common to a group of circuits, having the advantage of
lower cost per circuit. As shown, an inverter 33 is provided
induding a transformer 34 having a primary winding 35 and a
secondary winding 36. A voltage source 37 is connected between a
center tap of the primary winding 35 and a circuit point 38
connected to the emitters of a pair of transistors 39 and 40 having
collectors connected to end terminals of the primary winding 35 and
having base electrodes connected to a base drive circuit 42
operative to render the transistors 39 and 40 alternately
conductive. End terminals of the secondary winding 36 and a center
tap thereof are connected to a plurality of circuits 43-46 which
are of the same form. As shown, circuit 43 comprises a transformer
48 having primary winding and secondary winding 49 and 50. A center
tap of the primary winding 49 is connected through a resistor 51 to
one terminal of a source 52 of a signal voltage V.sub.1, the other
terminal of source 52 being connected to the center tap of winding
36. End terminals of the winding 49 are connected through diodes 53
and 54 to end terminals of the winding 36.
The center tap of secondary winding 50 is connected to a circuit
point 56 while end terminals thereof are connected through diodes
57 and 58 to an input of a low pass filter 60 the output of which
is connected to circuit point 56 through a resistor 61 and a source
62 of a signal voltage V.sub.2.
The operation of the circuit 43 is similar to the circuit of FIG.
3, variations in the magnitude of the voltage V.sub.1 of signal
source 52 producing changes in the amplitude of current pulses in
winding 49, producing corresponding changes in the amplitude of the
voltage in the secondary winding 50 and in the rectified and
filtered output at the output of the low-pass filter 60. Similarly,
changes in the magnitude of the voltage V.sub.2 change the
magnitude of current pulses in the secondary winding 50, thereby
affecting the magnitude of current pulses in the primary winding 52
to affect the voltage between the center taps of windings 49 and
36. Thus signals are passed in either direction through the
circuit. It is noted that the signal sources 52 and 62 are voltage
sources as diagrammatically illustrated but since there is always a
DC current in the same direction, therethrough, they can be in the
form of variable impedances such as a carbon type microphone, for
example, producing a modulation of the current and thereby a
variable voltage. Accordingly, the circuit 43 may be used to
interconnect a pair of terminal sets for bi-directional
transmission of signals therebetween with the signals being in the
form of either variable voltages or variable impedances.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 5 is a block diagram of a portion of a telephone system 64
incorporating interface circuitry according to the invention. Three
subsets 65-67 are illustrated, it being understood that a much
greater number may be employed. Subsets 65-67 are connected to
interface circuits 68-70 which are connected through lines 71-74 to
a common inverter 76, through lines 77-79 to a network 80 and
through lines 81-83 to a ringing and DC injection circuit 84.
Ring-trip circuits 85-87 are connected to the interface circuits
65-67 through pairs of lines 89 and 90, 91 and 92 and 93 and
94.
FIG. 6 is a circuit diagram of the interface circuit 68. Lines 71
and 72 supply a square wave signal from the inverter 76 at a high
frequency in relation to the highest frequency component of the
signals to be transmitted through the interface circuit 68. For
transmission of audio frequency signals, the frequency of the
square wave signal may be 200 KHz, by way of example. Lines 71 and
72 are connected through diodes 97 and 98 to a primary winding 99
of a transformer 100. Primary winding 99 has a center tap 101
connected through a capacitor 102 to the line 77 which is connected
through a resistor 103 to ground. Center tap 101 is also coupled to
ground through a synthetic inductor circuit 104 forming an
important feature of the invention as hereinafter described.
Transformer 100 has a pair of secondary windings 105 and 106
connected to diode bridge rectifier circuits 107 and 108 the
outputs of which are connected in series and to the input of a low
pass filter 110 formed by a pair of shunt capacitors 111 and 112
and a series inductor 113 with a load resistor 114 being connected
across the output of the low pass filter 110. Lines 115 and 116
connect the subset 65 and the output of the low pass filter 110, a
resistance 118 being connected in series with line 116, with
opposite ends of resistor 118 being connected through resistors 119
and 120, and lines 89 and 90 to the ring-trip circuit 85.
In operation, current pulses are applied alternately from lines 71
and 72 and through diodes 97 and 98 to the ends of the primary
winding 99, developing a square wave voltage across a primary
winding 99 and a corresponding square wave voltage across the
secondary windings 105 and 106 which are rectified, added and
filtered to produce a DC voltage across the resistor 114. In signal
transmission from the subset 65 to the network 80, a signal applied
between lines 115 and 116, either in the form of a change in
voltage or a change of impedance, produces a change in the current
in the secondary windings 106 and 105, reflecting a change in the
current in the primary winding 99 and developing a signal across
the resistor 103 which is applied through the line 77 to the
network 80. In signal transmission from the network 80 to the
subset 65, a signal applied on line 77 changes the current flow
through the two halves of the primary winding 99, effecting a
change in the square wave voltage as developed across the secondary
windings 105 and 106 and thereby a change in the voltage developed
across the resistor 114, applied through lines 115 and 116 through
the subset 65. It is noted that the synthetic inductor circuit 104
presents a high impedance at audio frequencies.
For ringing operation, a suitable AC signal, at a frequency of 20
Hz for example, is injected through line 81 to the synthetic
inductor circuit 104, developing a corresponding signal across the
resistor 114 which is applied through lines 115 and 116 to the
subset 65. A DC bias signal, if required, may also be applied
through line 81.
It is noted that the value of resistor 114 is such as to obtain
good ringing performance combined with reasonable ringing signal
drain. The value of resistor 103 is such as to provide the proper
input impedance, looking from the subset, which may be 900 ohms,
for example. In the network, 80, the line 71 may, for example, be
coupled through an audio hybrid circuit to a four-wire analog
network or to a digital network via appropriate converters.
DC current to the subset 65 is sensed by means of the series
current-detection resistor 118, the voltage developed thereacross
being applied through resistors 119 and 120 and lines 89 and 90 to
the ring-trip circuit 85. It is noted that the ringing potentials
are developed on an individual basis and both sides of the
current-detection resistor 118 can be near ground potential. Unlike
prior circuits, there is no large common mode voltage and no
difficulty with respect to isolation and the ring-trip circuitry is
simplified.
The synthetic inductor circuit 104 is an important feature of the
invention, eliminating the need for a large inductor of
conventional form and also being relatively inexpensive. The
circuit 104 comprises an operational amplifier 122 supplied with
negative and positive operating voltages from lines 73 and 74,
by-pass capacitors 123 and 124 being connected between ground and
the negative and positive supply voltage terminals of amplifier
122. The supply voltages on lines 73 and 74 may be minus 15 volts
and plus 15 volts, by way of example.
The output of amplifier 122 is connected to ground through three
series diodes 126 and is also connected to the base of a transistor
127 having an emitter connected to the base of a transistor 128 the
emitter of which is connected to a circuit point 129, connected
through a resistor 130 to ground. The collectors of the transistors
127 and 128 are connected to the center tap 101 of the primary
winding 99. The plus input of amplifier 122 is connected through
resistors 131 and 132 in series to the center tap 101 of winding 99
and is also connected through a resistor 133 to ground, a capacitor
134 being connected between ground and the junction between
resistors 131 and 132. The negative input of amplifier 132 is
connected to line 81, through a resistor 135 to ground and through
a resistor 136, to the circuit point 129.
For analysis of the operation of the circuit 104, assume that the
resistances of resistors 130, 131, 132, 133, 135 and 136 are
R.sub.A, R.sub.B, R.sub.C, R.sub.D, R.sub.E and R.sub.F,
respectively as indicated in FIG. 6; V.sub.C is the DC voltage
between ground and the collectors of transistors 127 and 128,
I.sub.C is the DC current flow to the collectors of transistors 127
and 128, v.sub.C is a small AC voltage superimposed on the DC
voltage V.sub.C ; i.sub.C is the AC current to the collectors of
transistors 127 and 128 resulting from the AC voltage v.sub.C.
Let G equal the forward gain from the plus input of amplifier 122
to the circuit point 129, equal to: ##EQU4##
It may be assumed that R.sub.B and R.sub.C are both relatively
large such that the current flow therethrough is negligible in
relation to I.sub.C.
With the illustrated circuit ##EQU5##
With respect to DC, the circuit therefore acts as a resistance
equal to: ##EQU6##
Analyzing the AC operation: ##EQU7##
Solving for the AC impedance, ##EQU8##
This is the equation for an inductor having an inductance:
##EQU9##
in series with a resistor having a resistance: ##EQU10##
It is desirable to introduce a non-linearity to limit output
current which is accomplished by the diodes 126 in the illustrated
circuit. Alternatively, a suitable diode or diodes may be connected
between ground and the plus input of amplifier 122 which would have
the advantage of being less subject to errors caused by the emitter
resistance of transistor 128.
Referring to FIG. 7, the inverter circuit 76 comprises an
oscillator 138 which supplies a high frequency signal, at 200 KHz
for example, to the primary winding 139 of a transformer 140 having
a pair of secondary windings 141 and 142 which have terminals
connected to the base electrodes of a pair of transistors 143 and
144. The other terminal of winding 141 is connected to the emitter
of transistor 143 and to ground while the other terminal of winding
142 is connected to a circuit point 146 which is connected to the
emitter of the transistor 144, through a capacitor 147 to ground
and through a capacitor 143 to a power supply terminal 150. The
collectors of transistors 143 and 144 are connected to terminals of
a pair of primary windings 151 and 152 of a transformer 154 having
a pair of center tapped secondary windings 155 and 156, the other
terminal of winding 151 being connected to the circuit point 146
and the other terminal winding 152 being connected to the power
supply terminal 150. In response to the signal from the oscillator
138, the transistors 143 and 144 conduct alternately through the
windings 151 and 152, generating square wave voltages in the
secondary windings 155 and 156.
The center tap of the secondary winding is grounded, the end
terminals thereof being connected through a pair of diodes 158 to
the line 73 to develop a negative DC voltage thereon and being
connected through a pair of diodes 158 to the line 74 to develop a
positive voltage thereon filter capacitors 159 and 160 being
connected between lines 73 and 74 in ground. The end terminals of
winding 156 are connected to lines 71 and 72 while the center tap
thereof is connected to ground.
Referring to FIG. 8, reference numeral 162 generally designates a
modified arrangement for signal transmission between a pair of
subsets 163 and 164. Subsets 163 and 164 are connected to rectifier
and filter circuits 165 and 166 each of which may include a pair of
diode bridge rectifier circuits like the circuits 107 and 108 and a
low pass filter circuit like the circuit 110. Circuit 165 is
connected to a pair of windings 167 and 168 of a transformer 170
while circuit 166 is connected to a pair of windings 171 and 172 of
a transformer 174. Additional windings 175 and 176 of transformers
170 and 174 have end terminals connected through pairs of diodes
177 and 178 to end terminals of a secondary winding 179 of a
transformer 180 having primary windings 181 and 182 connected to a
river circuit 183 supplied with a signal from an oscillator 184.
Driver circuit 183 may have circuitry similar to that in the
inverter 76 shown in FIG. 7.
The center tap of the winding 179 is connected to ground through a
synthetic inductor circuit 186 which may have a circuit like that
of the circuit 104 of FIG. 6. The center taps of windings 175 and
176 are connected to ground through synthetic capacitor circuits
187 and 188 which ringing signals may be applied through lines 189
and 190.
The operation of the circuit arrangement of FIG. 8 is similar to
that of the arrangement of FIGS. 5-7. In the arrangement of FIG. 8,
the synthetic inductor circuit operates to provide a high impedance
in common for two interface circuits, whereas in the circuit
arrangement of FIGS. 5-7, there is one inductor circuit associated
with each interface circuit.
FIG. 9 is a circuit diagram of the synthetic capacitor circuit 187,
the other circuit 188 being the same. The circuit 187 comprises an
operational amplifier 192 supplied with negative and positive
operating voltages from a suitable source, by-pass capacitors 193
and 194 being connected between ground and the negative and
positive supply voltage terminals of amplifier 192. The supply
voltages on the lines connected to capacitors 193 and 194 may be
minus 15 volts and plus 15 volts, by way of example.
The output of amplifier 192 is connected to ground through three
serie diodes 196 and is also connected to the base of a transistor
197 having an emitter connected to the base of a transistor 198 the
emitter of which is connected to a circuit point 199, connected
through a resistor 200 to ground. The collectors of the transistors
197 and 198 are connected to the center tap of the primary winding
175. The plus input of amplifier 192 is connected through a
resistor 201 and a parallel capacitor 202 to the center tap of
winding 175 and is also connected through a resistor 203 to ground.
The minus input of amplifier 192 is connected to line 189, through
a resistor 204 to the circuit point 199 and through a resistor 205
to ground.
The circuit of the synthetic capacitor 187 is similar to that of
the synthetic inductor 194, differing therefrom in that the
parallel combination of the resistor 201 and the capacitor 202 is
connected between the collectors of transistors 197 and 198 and the
plus input of amplifier 192, instead of the arrangement with the
series resistors 131 and 132 and the shunt capacitor 134 of the
circuit 104. The operation is generally the same, however, a
phase-shifted signal being applied to the plus input of the
amplifier 192 in a manner such as to obtain synthetic capacitor
operation. It is noted that in the circuit 187, a ringing signal or
a DC bias signal, if desired, can be readily injected into the
circuit at the minus input of the amplifier 192 as illustrated.
It will be understood that modifications and variations may be
effected without departing from the novel concepts of this
invention.
* * * * *